In his first-ever interview, the sole Australian survivor of the Waco siege has revealed to 60 Minutes what really took place during the 51-day stand-off, and why he believes crazed cult leader David Koresh will one day return to Earth as the son of God.

Asked to clarify a remark he made after Monday's opening of talks about not making concessions to Britain, he played it down as a statement of the obvious not to concede arguments at the outset and all the more so because his meeting with Britain's David Davis in Brussels did not launch talks on substance.

"My mandate is to defend the single market, defend the European Union, which the United Kingdom has decided to leave," Barnier told reporters after briefing ministers from EU member states in Luxembourg. "I'm well aware that throughout the process there will be points of compromise."

The Union wanted a "fair deal", he said, renewing a vow not to punish Britain or take "revenge" for a withdrawal that has rocked the bloc and poses huge challenges for negotiators.

EU officials and diplomats greeted the agreement of Brexit Secretary Davis to a format and sequencing of talks that had been proposed by Barnier as sign that British Prime Minister Theresa May's weakened and potentially divided government was bending to Brussels' insistence on how negotiations will work.

"A very good day -- for us," one senior EU figure said with a note of triumph after discussions finally began, nearly a year after the Brexit vote and with only 21 months to reach a deal.

In public, EU negotiators have kept their satisfaction in check, however, and Barnier was at pains to sound conciliatory.

But in Britain many media portrayed Davis, a veteran Brexit campaigner, as having climbed down from his insistence that the EU must immediately open talks on a future free trade agreement rather than limit negotiations, as they are now, to basic issues of the divorce, such as the rights of expatriate citizens.

Germany's Europe minister, Michael Roth, said in Luxembourg that the opening of the negotiations had shown British leaders still needed a "reality check" on what they could achieve.

Barnier made clear on Tuesday that while there must be a first phase that excludes trade discussions, he does hope that four weeks of monthly negotiating sessions can get him to the point where he can ask EU leaders in October to let him move on and discuss how the two sides can keep commerce open from 2019.

At the same time, while it was a priority, he said, to show progress in averting difficulties for troubled Northern Ireland as a result of being on a new EU-UK border, agreement on how the border would work would depend on wider future trade ties.